For the purposes of discussion, the term “fingerprint” will mean the skin surface friction ridge detail of a single fingerprint, partial fingerprint or any portion of the skin surface friction ridge of up to and including the entire hand. Fingerprint information has been collected from human fingers by means of ink and paper. In recent years various electronic fingerprint scanning systems have been developed utilizing optical, capacitance, direct pressure, thermal and acoustic methods. Optical systems often employ CCD (charge coupled device) arrays to detect light reflected from the surface being imaged. Capacitance systems often inject a radio frequency signal into the skin/tissue and image the finger by reading the attenuated radio frequency signal radiating from the fingerprint ridges. Direct pressure systems often utilize an array of pressure sensing elements in contact with the finger and map a pressure gradient field. Thermal systems often detect temperature differences between air trapped between ridges of a fingerprint and the skin corresponding to the ridges in order to produce a thermal image.
Acoustic or ultrasound scanning devices and methods often employ a piezoelectric transducer that sends a wave or pulse through an acoustic transmitting media. The pulse then partially reflects back at each media interface. The reflected pulse is detected by a sensing element in the transducer. By knowing the time that lapsed between sending and detecting the pulse, the distance traveled by the pulse for each reflecting surface may be determined. Range gating (biasing) this signal in time may be used to know which reflected signals correspond to the fingerprint, and which do not. By knowing the time that lapsed between sending and receiving, it is possible to determine whether a ridge of the fingerprint or a valley of the fingerprint reflected the signal. Ridges in the fingerprint will have a shorter lapsed time than valleys in the fingerprint. By knowing whether a ridge or a valley reflected the signal, and by knowing the position of the transducer, an image of the friction-ridge may be created.